The accurate control of fluid flow in microfluidic devices is key to the ability to perform assays, for example diagnostic immunoassays in a microfluidic device. Fluid flow can be achieved with actuated or passive microfluidics. Actuated microfluidics control fluid flow using an external power source or pump. In passive microfluidics, fluid flow is encoded by the design of the microfluidic device itself rather than any externally applied forces, with fluid flow occurring due to capillary forces. Passive microfluidic devices are attractive due to their low power consumption, portability and low dead volume.
The optimization of performance of assays conducted in microfluidic devices is dependent on control of the timings of fluid residence in various portions of a microfluidic channel, where various assay steps are performed. Frequently, this is controlled by the incorporation of structural delay elements, such as delay loops that take a controlled period of time to fill. In addition, fluid composition itself affects flow characteristics. For example, the use of surfactants provides surface wetting properties that allow fluid flow in a microfluidic channel under passive capillarity.
In general, a higher surfactant concentration leads to a faster flow rate and consequently a shorter residence time within a microfluidic channel. However, a problem is observed for very narrow channels, which have a high surface area to volume ratio. As a fluid front moves along such a channel, its surfactant content becomes depleted as the channel/fluid interface is continually coated with surfactant. This depletion of surfactant content results in a gradual slowing of fluid flow along the channel. Increasing the surfactant concentration to compensate for this can give erratic fluid flow rates which are too fast to provide precise control and are vulnerable to non-uniform filling patterns, incomplete wetting of channel walls and air bubble formation if there are even minor variations in dimensions or surface irregularities from chip-to-chip. In addition, the flow rate of fluid within a microfluidic channel is very sensitive to small changes in concentration of surfactant. Differences in flow rate give rise to poor precision of tests carried out in a microfluidic device. In a commercial setting, low-cost (e.g. injection moulded from thermoplastic polymer) microfluidic components are required and a means of improving the consistency of fluid flow is necessary in order to achieve acceptable test precision.
It has now been determined that these problems can be overcome by using a dual flow control reagent mixture that provides uniform wetting of microfluidic channel walls and predictable fluid flow rate.